Some web sites and other types of interactive systems implement recommendation services for generating personalized recommendations of items stored or represented in a data repository. One common application for recommendation services involves recommending products for purchase, rental, subscription, viewing or some other form of consumption. For example, some e-commerce web sites provide services for recommending products to users based on their respective purchase histories, rental histories, product viewing histories, or item ratings. Recommendation services are also used to recommend web sites, news articles, users, music and video files, and other types of items.
Recommending content to users has many benefits, but can also lead in some cases to negative user experience with the recommendation service, for example when the recommended content is offensive. Users frequently opt-out of receiving e-mail advertisements, complain, or quit using systems implementing the recommendation services altogether if they receive recommendations for content they deem offensive or obnoxious. What offends or annoys users is variable from user to user and also can vary for a particular user over time.